Citrix has released a good document on how to customize the Receiver for Web User Interface.
Receiver for Web provides a simple mechanism for customizing the user interface. Each Receiver for Web site includes a contrib folder containing files in which UI customizations can be placed. The contents of this folder are preserved when StoreFront is upgraded to a newer version. However, page elements may be added, removed, or renamed in future releases so there is no guarantee that specific customizations will continue to function correctly following an upgrade, especially when the UI changes significantly between releases.
Download or read the entire PDF here.